The present invention relates to an ignition timing control device for an engine which detects the occurrence of knocking in the engine and controls the ignition timing accordingly.
In general, a conventional ignition timing control device of this type employs a feedback control system in which an ignition signal, produced with a timing based on a preset reference ignition timing characteristic, is delayed in phase by a certain angle or an angle corresponding to knocking strength whenever engine knock occurs. On the other hand, if no engine knock is present, the amount of phase shift is decreased with a predetermined time constant.
In the feedback control system, the angle of delay from the reference ignition timing is utilized to suppress engine knock, and accordingly the reference ignition timing must be set at a point advanced in phase from the knock limit of the engine. Therefore, at the edge of the control range, the ignition timing is always advanced from the knock limit, thus resulting in a high level of engine knock. Accordingly, it is desirable that the reference ignition timing be set slightly advanced from the knock limit.
However, it is practically impossible to do so for all operating conditions of the engine, and it is unavoidable for the reference ignition timing to lag the ignition timing which corresponds to the knock limit. Ignition thus occurs later than for the optimum ignition timing of the knock limit, and hence it is impossible to provide optimum ignition timing control under all operating conditions.